Yard and Garden: Effect of extreme cold on trees, shrubs

AMES — Gardeners may be wondering just how, and if, trees, shrubs and perennials survive below-zero temperatures.

They know that the below-ground parts of a tree are kept insulated by a layer of snow, and that is important to winter survival, but the exposed parts of a tree are not so protected. Horticulturists with Iowa State University Extension and Outreach explain the cold hardiness of trees, shrubs and perennials.

To have more questions answered contact Hortline at 515-294-3108 or hortline@iastate.edu.

What effect does the timing of extremely cold temperatures have on trees and shrubs?

Timing is very important. The cold hardiness of woody plants gradually increases in fall with the maximum cold hardiness occurring in mid-winter (mid to late January). The cold hardiness of woody plants slowly decreases from mid-winter to spring. A temperature reading of -20 degrees Fahrenheit in late November or early March could be quite destructive while the same temperature in mid-winter would cause little or no harm to trees and shrubs that are reliably cold hardy in Iowa.

Our temperature dropped to 20 degrees below zero. What effect will the cold temperatures have on trees and shrubs?

Trees and shrubs that are native to Iowa (or similar regions of the world) are well adapted to our climate and should have suffered little or no damage. However, marginally hardy plants, such as Japanese maple (Acer palmatum), flowering dogwood (Cornus florida) and Japanese flowering cherry (Prunus serrulata) may have sustained damage. The maximum cold hardiness of most Japanese maple, flowering dogwood and Japanese flowering cherry cultivars is -20 degrees Fahrenheit. Damage may vary from the dieback of twigs and branches to the complete death of the tree.

The cold temperatures also may have destroyed the flower buds on flowering quince (Chaenomeles spp.) and some forsythia cultivars, such as ‘Lynwood Gold’ and ‘Spring Glory.’ As a result, these shrubs will likely produce few, if any, flowers in spring. Fortunately, the cold temperatures should not have any long term effects on these shrubs. The leaf buds on flowering quince and forsythia are hardier than their flower buds. The shrubs should leaf out normally in spring.

The cold temperatures should have little impact on the flowering of most trees and shrubs. Crabapples, lilacs, viburnums, dogwoods and spireas possess excellent cold hardiness and should bloom normally this spring. Forsythia cultivars ‘Meadowlark’ and ‘Northern Sun’ should also bloom well as their flower buds can tolerate temperatures to -30 degrees Fahrenheit.

Our temperature dropped to 20 degrees below zero. What effect will the cold temperatures have on fruit trees?

The cold temperatures likely damaged peach and sweet cherry trees. Peach trees are not reliably cold hardy in much of Iowa. Temperatures below -18 degrees Fahrenheit will destroy the flower buds on peach trees. Temperatures of -25 degrees or below may damage the peach trees themselves. Damage may vary from the dieback of twigs and branches to complete death. The flower buds on some sweet cherry cultivars are slightly more cold hardy than those on peaches and can survive temperatures down to -20 degrees Fahrenheit. Temperatures below -25 degrees may result in branch dieback or the death of some sweet cherry trees.

Iowa gardeners should expect poor crops on peaches and sweet cherries this summer. Additionally, some trees may suffer extensive branch dieback or death. On a positive note, the cold temperatures should not have damaged most apple, pear and sour (tart) cherry trees.

Our temperature dropped to 20 degrees below zero. What effect will the cold temperatures have on perennials?

If covered with snow, the cold temperatures should not have harmed the perennials. Snow is an excellent insulator and protects covered plants. Perennials without a protective layer of snow may have suffered serious damage to their crowns and roots, possibly resulting in the death of some plants.